Union workers at the nation’s only uranium conversion plant, in Metropolis, Ill., have erected 42 crosses nearby in memory of workers who died of cancer. Twenty-seven smaller crosses symbolize workers who have survived the disease.

The memorial is a fitting backdrop for the contentious labor dispute that has shaken Metropolis — the self-proclaimed hometown of Superman, which sits on the Ohio River at the southern edge of Illinois. Many workers believe that the plant contributed to their fellow employees’ illnesses, which is a central reason the union is refusing to accept the plant operator’s plan to reduce pensions for newly hired workers and health benefits for retirees.

On June 28, Honeywell, the plant operator, locked out its 220 union employees after negotiations stalled, accusing the union of refusing to give the company 24 hours’ notice of a strike. The union has picketed ever since.

“We deal with hydrofluoric acid,” said Darrell Lillie, president of United Steelworkers Local 7-669, which represents the union workers. “We make fluorine. This is bad stuff. The least we feel like we could have is good medical benefits when we retire.”

Honeywell rejects the notion that its facility makes workers more susceptible to cancer and maintains that its labor proposal would raise the average overall salary for employees. The company says that is fair given that the plant, which converts milled uranium into uranium hexafluoride for nuclear reactors, is projected to lose $20 million this year.

“Unfortunately, the union has demonstrated very little desire to reach a mutually beneficial and workable agreement that acknowledges the economic realities of the plant,” said Peter Dalpe, a spokesman for Honeywell.

In Metropolis, a blue-collar community of 6,500, Clark Kent has long been the main draw. A 15-foot tall Superman keeps solemn watch outside the county courthouse, and the local newspaper is called The Metropolis Planet. The Chamber of Commerce used to hand out packets of “kryptonite” candy to children.

But this summer everyone is concerned about uranium, not kryptonite.

Workers have long feared that the plant poses dangers. According to Honeywell’s quarterly financial filings, the Environmental Protection Agency and the Justice Department are investigating whether the plant complied with federal law in its storage of sludge. A grand jury has been convened in the matter, the filings said.

Mr. Dalpe said Honeywell was cooperating with the E.P.A. investigation, “which resulted from a company self-disclosure more than two years ago.”

Federal inspectors have been on-site monitoring the situation, said Roger Hannah, a spokesman for the Nuclear Regulatory Commission. “At this point, we’re satisfied they’ve met all safety regulations, and that they have a continued emphasis on safety,” he said.

In December 2003, an accidental release of uranium hexafluoride sent a plume of gas into the air, and nearby homes were evacuated. The Illinois Environmental Protection Agency sued Honeywell over the accident, and the Nuclear Regulatory Commission issued two safety violations, which led to increased oversight.

With the union workers locked out, 152 salaried Honeywell employees have been running the plant and 203 contract workers were brought in from a Louisiana company to help.

“They’re trying to bring in old recruits who have been retired and train them to run the plant,” said Jerry Baird, whose restaurant, Diamond Lil’s, has been hauling barbecue, lemonade and ice to the picket line. “If they remember everything, it’ll probably run. If they don’t, they’ll probably kill us all.”

Mr. Dalpe said Honeywell was not aware of any evidence that pointed toward a higher rate of illness at its Metropolis plant than other facilities.

“Publicly available and independent government health data, including from the Centers for Disease Control, demonstrate conclusively that the cancer incidence rates in counties where our Metropolis employees reside are at or below those of the state of Illinois and the U.S. over all,” he said.

But because the plant, then called Allied Chemical, produced atomic material for the cold war effort, it is part of the federal government’s Energy Employees Occupational Illness Compensation Program.

Under the program, the government has paid out $15.2 million in compensation and medical benefits to former workers at the plant or their families. Those workers were determined to have become ill from radiation exposure while working between 1959 and 1976 or suffered residual radiation exposure between 1977 and 2009.

Mr. Dalpe said those payments included claims from more than 30 years ago and do not “reflect health and safety practices at our Metropolis Works Plant, nor do they indicate a high level of risk for our current employees.”

Meanwhile, labor negotiations are scheduled to resume on Tuesday. But union workers and their families continue to picket around the clock, and a rally was held Saturday.

“In Metropolis, there are friendships and personal ties between company and union employees,” said Billy McDaniel, the town’s mayor. “It makes it hard, because it splits those relationships. We certainly don’t want to lose Honeywell. And we don’t want to lose any of our workers.”